Evaluation of Galvanic and Stray Current Corrosion in 70/30 Copper-Nickel/Alloy 625 Piping Systems.

Abstract : Because of the difficulty in ensuring full-time electrical isolation of dissimilar metal piping, alternate methods of coping with galvanic corrosion must be used. Use of short, electrically isolated piping sections between dissimilar metals can reduce galvanic corrosion by increasing the electrical resistance of the seawater path through which the galvanic current must flow. The objective of this project was to determine the magnitude and distribution of galvanic corrosion of 70/30 copper-nickel piping when coupled to alloy 625 piping, to determine the efficacy of various lengths of isolated separator pipes made of either alloy, and to determine the amount of stray current corrosion that could occur on the separator piping as a function of pipe material. The presence of separators of any length in 2-in. pipe lowered the amount of galvanic corrosion between copper-nickel and alloy 625, with a 50- to 60-percent reduction in metal loss using 3-ft (1-m) separators. A further reduction of another 30 to 50 percent was achieved by increasing the separator length to lOft (3 m). The use of copper-nickel separators generally resulted in higher metal losses than the use of alloy 625 separators. The effect of the separator was maximum under low flow conditions. Alloy 625 separators were more effective than copper-nickel separators. Smaller diameter copper-nickel pipe experienced higher corrosion rates than pipe of equal diameter to the alloy 625 pipe to which they were coupled. Linearity of potential profile through the separators was an accurate indicator of whether stray current corrosion was taking place. The Navy should use 3-ft (l-m) titanium separator pipes, electrically isolated at both ends, to minimize galvanic corrosion in 2-in.-diameter dissimilar metal seawater piping joints. (AN)